Cross-Site Request Forgery

5-minute read
Paul Reardon
Technical • Security

Cross-Site Request Forgery (XSRF or CSRF), often pronounced as “Sea-Surf”, attacks are a serious threat to web applications. They exploit the trust a website has in a user’s browser, tricking the browser into sending unauthorised requests to the website. This can lead to data breaches, unauthorised actions on a user’s account, and other malicious activities.

The Attack Vector

A CSRF attack typically unfolds in the following way :

  1. The user logs into a legitimate website.
  2. The attacker lures the user to a malicious website. This can happen through phishing emails, social engineering, or even compromised advertisements on legitimate sites.
  3. The malicious website contains a hidden form or JavaScript code that constructs a request to the legitimate website. This request is crafted to perform a specific action, like changing the user’s password or initiating a funds transfer.
  4. When the user interacts with the malicious website (e.g., clicks a link or loads an image), the hidden request is automatically sent to the legitimate website.
  5. The legitimate website, seeing the request coming from the user’s authenticated browser, processes the request, unaware that it originated from a malicious source.

Impact of a CSRF attack

The consequences of a successful CSRF attack can be severe:

  • Data Loss or Corruption : User data can be modified or deleted without their consent.
  • Unauthorised Actions : Actions like purchases, password changes, or data sharing can be performed without the user’s knowledge.
  • Financial Loss : In the case of financial applications, CSRF can lead to unauthorised transactions and financial loss.
  • Reputation Damage : A successful attack can damage the reputation of the website and erode user trust.

Real World Examples

  1. Gmail CSRF Vulnerability (2007) : A CSRF vulnerability was discovered in Gmail that allowed attackers to add email filters, forward emails, and even delete emails from a victim’s account without their knowledge. This incident highlighted the potential for CSRF to impact even major web applications.

  2. ING Direct CSRF Vulnerability (2008) : ING Direct, a major online bank, faced a CSRF vulnerability that enabled attackers to transfer funds from victim’s accounts without their authorisation. This incident underscores the financial implications of CSRF attacks.

  3. YouTube ‘Like’ Button Exploit (2008) : A CSRF vulnerability in YouTube’s ‘Like’ button functionality allowed attackers to trick users into liking videos without their consent. While seemingly less severe, this incident demonstrates how CSRF can be used to manipulate user behavior and spread misinformation.

  4. Netflix Account Takeover (2014) : A CSRF vulnerability in Netflix allowed attackers to take over user accounts, change passwords, and modify account details. This attack illustrates how CSRF can be used for identity theft and unauthorised access to sensitive information.

  5. McAfee Network Security Platform CSRF Flaw (2017) : A CSRF vulnerability in McAfee’s Network Security Platform allowed attackers to change device configurations and potentially compromise the security of entire networks. This case demonstrates the potential for CSRF to extend beyond individual user accounts and impact critical infrastructure.

XSRF tokens to the rescue

XSRF tokens are a powerful defense mechanism against these attacks. They act as a unique, unpredictable value that’s included with every legitimate request. This ensures that only requests originating from the actual web application itself are considered valid.

The tokens work as follows :

  1. Token Generation :
    • When a user visits a web application, the server generates a unique XSRF token.
    • This token is typically stored in a secure location, such as a server-side session or a HttpOnly cookie (which prevents JavaScript from accessing it).
    • The token is also embedded within the HTML forms or included as a custom header in AJAX requests sent to the client.
  2. Token Submission :
    • When the user submits a form or makes an AJAX request, the XSRF token is sent back to the server along with the request data.
  3. Token Validation :
    • The server compares the submitted token with the one stored in the user’s session or cookie.
    • If they match, the request is deemed legitimate and processed.
    • If they don’t match, the request is rejected as a potential XSRF attack.

Implementation Consideration

XSRF token implementation varies slightly depending on the web development framework or technologies you’re using. Here’s a general outline :

  • Server-Side :
    • Generate a cryptographically secure random token.
    • Store the token in a secure location (session, HttpOnly cookie).
    • Include the token in HTML forms as a hidden field or in AJAX requests as a custom header.
    • Validate the token on the server for every state-changing request.
  • Client-Side :
    • Include the token in all relevant forms and AJAX requests.
    • Ensure the token is sent back to the server with each request.

Additional Defense Strategies

While XSRF tokens are highly effective, combining them with other security measures creates a more robust defense :

  • SameSite Cookies : The SameSite attribute for cookies helps prevent CSRF attacks by restricting cookies from being sent in cross-site requests.
  • Checking the Referer Header : While not foolproof, examining the Referer header can help identify requests originating from different domains.
  • CAPTCHA for Sensitive Actions : Employing CAPTCHA for critical actions adds an extra layer of protection.
  • User Education : Educating users about the risks of CSRF and the importance of avoiding suspicious links and websites can help prevent attacks.

Conclusion

Cross-Site Request Forgery attacks represent a persistent and potentially devastating threat to web applications and their users. As demonstrated by the real-world examples highlighted in this article, the consequences of a successful CSRF attack can range from data breaches and unauthorized actions to financial loss and reputational damage.

However, by understanding the mechanics of CSRF and implementing robust defense mechanisms like XSRF tokens, alongside other security best practices, developers can significantly reduce the risk of these attacks. The proper implementation of XSRF tokens, combined with measures such as SameSite cookies, Referer header checks, and user education, create a multi-layered defense that safeguards web applications and protects user data.